This invention relates generally to an apparatus for reducing nitric oxide (NOx) emissions in gas turbine combustion system. More specifically this invention relates to the apparatus and methodology for assembling the reduced NOx combustion system.
Combustion liners are commonly used in gas turbine engines to provide a protected environment for compressed air and fuel to mix and react in order to generate the hot gases necessary to drive a turbine. Due to the extreme exposure to elevated temperatures, combustion liners are typically coated with a thermal barrier coating (TBC) to reduce effects of high temperatures on the base metal of the combustion liner. Thermal barrier coating is typically comprised of a metallic bond coat and a ceramic top coat. Over time, exposure to elevated temperatures causes the TBC to erode and therefore necessitates inspection of the combustion liner and repair and replacement of worn liner elements, Including the thermal barrier coating. Typically, combustion liners can undergo multiple inspection and repair cycles at regular intervals before replacement of the combustion liner is required.
In some combustion liners additional hardware is present that must be removed before overhaul and maintenance function can be performed on a combustion liner. Examples include supports for fuel injectors or internal combustion chamber walls. More specifically, a two-stage combustion liner known in the art of gas turbine combustors used for power generation, includes a venturi that separates a first combustion chamber from a second combustion chamber. This venturi must be removed in order to repair and overhaul the surrounding combustion liner. An example of a prior art venturis installed within two-stage combustion liners are shown in FIGS. 1 and 2 and are described in detail in U.S. Pat. Nos. 5,117,636 and 6,446,438, respectively.
While two stage combustion systems are well known in the field of combustion technology, enhancements have been made to further reduce NOx emissions over prior art combustors. Early two stage combustors contained combustion liners with venturis similar to that disclosed in FIG. 1 and described in U.S. Pat. No. 5,117,636, where compressed air is employed to cool venturi walls 32 and exits passageway 44 in the downstream direction. Further development of two stage combustion technology led to a counter-flow venturi technology, as disclosed in U.S. Pat. No. 6,446,438 and shown in FIG. 2, where cooling air was redirected into a premix chamber 12 after cooling venturi assembly 11 and utilized in the combustion process. While this technology has advanced combustion systems to produce lower NOx emissions, it has created a configuration that demands a unique overhaul and repair sequence. The present invention provides a combustion liner configuration with improved assembly and disassembly methodology that results in improved inspection and repair techniques.